Most pond problems can be prevented by proper pond construction, and maintenance. However, not all pond owners are lucky enough to have participated in the design and construction of their pond, or its management and maintenance through the ponds entire history. Bottom aeration can turn this problem around. To speed up this process you can keep leaves, and other debris from entering your pond. You can also cut and remove weeds and algae. It would be best to remove weeds after they mature, as you will be removing the nutrients they have absorbed from the pond.
Pond Water Circulation

Winds can easily mix bodies of water down to around 8' deep in early spring and in late fall as the water becomes a uniform temperature top to bottom. As the temperature rises in the spring, the water on the surface becomes lighter and cannot mix with the colder and heavier water on the bottom. This creates a lack of oxygen in the bottom water.

Bottom Aeration

Adding oxygen through a diffuser in the bottom of the pond creates a water column of fine bubbles rising to the surface. This action pulls cold stagnant water to the surface where it is aerated by oxygen producing phytoplankton, and then circulated back into the bottom. Getting oxygen into the bottom allows aerobic bacteria, and all other life to exist in the bottom. The ponds, Eco system becomes balanced top to bottom allowing for much faster decomposition of sludge, and weed control. Fish will grow faster, as more food is available. Bottom aeration will also keep an open area in the ice allowing light to penetrate and keep micro algae alive to produce more oxygen. Snow cover on ice can cause all micro algae to die.

Decomposition

As bacteria decompose the bottom sludge, (weeds, leaves, etc.) they use a large amount of oxygen. As bottom sludge builds up, the oxygen demand becomes even greater. It can become greater than the amount of oxygen available.

The loss of oxygen in the bottom causes the beneficial oxygen using bacteria to suffocate and die. When this occurs, they are replaced by Anaerobic bacteria which requires no oxygen. These Anaerobic bacteria decompose 20 to 30% slower and produce gasses such as Methane, and Hydrogen Sulfide causing a black smelly septic bottom in your pond. Anaerobic decomposition, being slower, also allows for a faster buildup of muck, and releases "Phosphors", which is "fertilizer", now available to grow more weeds and algae.